Recently I demonstrated a prototype blade to an IT admin from a large bank. He'd worked with lots of different HP server models over the years, but a bright red color peeking out from the server blade chassis caught his eye. I popped off the cover and showed him the motherboard. It was coated with a glossy red solder mask that made it shine like a candy apple.
"Why is it red?" he asked. "Doesn't red color mean hot plug?"
"Well," I answered, "Not in this case. We do use dark red handles on stuff that's hot pluggable, like power supplies." (By the way, our official description for the hot-pluggable color is 'port wine', not red.)
The bright red color, I explained, signified that this particular motherboard was a Proto-2, aka a second-generation prototype. When the product went to production, the circuit boards would all be green. Until then, the various stages of test hardware would be built in different colors.
For many years the ProLiant team has used other colors of solder mask and silkscreen during development. (The solder mask, also called solder resist, is a paste-like substance that coats most of the outer layers of a circuit board, giving it its color. By 'silkscreen' I mean the letters that are printed on the circuit board to reference components.)
There is no set rule, but we tend to use brighter red, orange, and yellow colors for the earliest prototypes, with darker purples, blues, and greens for later ones.
Not all silkscreen colors work well with solder mask colors. Allen Shorter, another ProLiant engineer here at HP, told me he learned (through experience, unfortunately) that white silkscreen on yellow solder mask makes the lettering really, really hard to read.
There are apparently lots of theories for why green became the normal circuit board color. (I don't buy into the theory that it's because green is the easiest color for the human eye to see -- PCBs aren't supposed to be on display.) It's simple enough for PCB fabs to make other colors, but it's slighly more time- and cost-effective for them to stick with a single color, so for large-scale production most everyone uses green.
One of the BEST videos I've seen in awhile about a very unsexy story - servers. Steve Brown at Intel did this and it's great. Even your mom will understand servers after seeing this.
Have a great weekend!